1. Field of the Invention
The present invention relates to a projection type image display device, in particular, the projection type image display device which operates to magnify an image appearing on a liquid crystal display and project the magnified image onto the screen.
2. Description of the Related Art
FIG. 10 shows an optical arrangement of a projection type image display device according to a first related art of the invention. The projection type image display device is arranged to have a reflective type liquid crystal light valve provided with a photoconductive layer. As shown, this type display device includes a CRT 112 for displaying an original image, a reflective type light valve 103 for forming and holding an image for the displayed image, a lens 113 located between the CRT 112 and the light valve 103, a light source 104 for applying a ray, a polarizing beam splitter 106 for applying the light from the light source 104 and passing specific polarized components of the light (reflected light) from the light valve, a lens 105 located between the light source 104 and the polarizing beam splitter 106, a projecting lens 107 for receiving the light passed through the polarizing beam splitter 106 and magnifying the image formed of the light, and a screen 108 on which the projected image is formed.
When the image is displayed on the projection type image display device, the image displayed on the CRT 112 is applied to the reflective type liquid crystal light valve 103 through the lens 113 and the light source applies light of the liquid crystal light valve 103.
FIG. 11 is a section showing an arrangement of the reflective type liquid crystal light valve 103. As shown, the light valve 103 is composed of a pair of glass substrates 115a, 115b, transparent electrodes 116a, 116b, and a photoconductive layer 117. The transparent electrodes 116a and 116b are formed of transparent conductive films (ITO) on the glass substrates 115a and 115b, respectively. Then, amorphous silicon hydride (a-Si:H) is formed on the transparent electrode 116a and is served as the photoconductive layer 117. The amorphous silicon hydride (a-Si:H) is formed as the photoconductive layer 117 on the transparent electrode 116a. The amorphous silicon hydride is formed of silane gas and hydrogen gas by means of a plasma CVD method. A multilayered film of TiO.sub.2 and SiO.sub.2 is formed on the photoconductive layer 117 by means of the sputtering method. On the multilayered film, the polyimide films are spin-coated as orientation films 119a and 119b. Then, the molecular orientating treatment is done on the orientation films 119a and 119b by means of the rubbing technique. The resulting glass substrates 115a and 115b are pasted with a spacer 120 laid therebetween.
Mixing nematic liquid crystal having a chiral material added thereto is injected and sealed between the glass substrates 115a and 115b. This serves as a liquid crystal layer 121. The resulting composition is a hybrid field-effect mode reflective type liquid crystal light valve 103. The light valve 103 uses as an operating mode a vertical ECB mode or a guest host (GH) mode.
Between the transparent electrodes 116a and 116b included in the light valve 103 arranged as above, there is applied a voltage from an A.C. power source 122. When the image from the CRT 112 enters from the glass substrate 115a, the impedance of the photoconductive layer 117 changes depending on the quality of incident light. With this change, the voltage applied to the liquid crystal layer 121 is changed, thereby changing the orientation of the liquid crystal, so that the image corresponding to the image from the CRT 121 may be formed on the liquid crystal layer 121.
The light from the light source 104 enters into the reflective type liquid crystal light valve 103 on which an image is formed through the lens 105 and the polarizing beam splitter 106, the incident light is reflected on a dielectric mirror 118 composing the light valve 113. Since the reflected light is passed through the portion of the liquid crystal layer 102 whose orientation, the reflected light changes its polarizing direction through the electro-optical effect. Hence, the selected reflected portion is allowed to be passed through the polarizing beam splitter 106.
This reflected light is magnified through the effect of the projective lens 107. The image formed on the light valve 103 is projected onto the screen 108. In turn, the description will be oriented to an optical arrangement of a projection type image display device according to the second related art of the invention with reference to FIG. 12. This second related art is analogous to the first related art. Hence, the corresponding components have the same reference numbers. As disclosed in Japanese Patent Lying Open No. Hei 4-181226 or Hei 4-204919, the second related art is arranged so that a light source 123 may apply light to a transmittance display panel 125 and the light L1 passed through the panel 125 may form an image on the reflective type liquid crystal light valve 108.
The use of the transmittance type display panel makes it possible to reduce the image display device in size. Recently, a high-resolution transmittance type display panel is now developed. The transmittance type display panel 125 used in the second related art does not operate to be luminous but the transmittance of the display panel 125 is changed on the driving signal so that the display panel 125 may modulate the intensity of the light from the light source provided in another light source for displaying an image or character. In this related art, several displays having light-passivation ceramics have been proposed such as a liquid crystal display panel, an electrochromic display, or a PLZT. In particular, the liquid crystal display panel is widely used for a pocket-sized TV (Television), a wordprocessor, or a projector. It is substantially completed.
FIG. 13 shows an active-matrix liquid crystal panel as an example of a transmittance display panel 125. The liquid crystal panel is composed of a pair of opposite substrates 128a and 128b, a spacer 128 for keeping an interval between these opposite substrates, a liquid crystal layer 127 sealed between the opposite substrates 126a and 126b, a switching element 129, a pixel area 130, both of which are formed on the opposite substrate (TFT substrate) 126b, and a light cut-off layer 110 formed on the opposite substrate 126a and having an opening for the pixel area.
In the foregoing arrangements, when an image displayed on the transmittance display panel is written in the photoconductive layer 117 of the reflective type liquid crystal light valve 103, the thickness of the glass substrate brings about a parallax, thereby making the image vague and lowering resolution. To cope with these shortcomings, it is necessary to form the overall display screen onto the photoconductive layer through the effect of just one lens. However, this arrangement makes the writing optical system larger in size.
To keep the image clear, the overall image of the transmittance display panel is focussed on the photoconductive layer through the effect of one lens. This method enlarges the writing optical system.
Further, the Japanese Patent Lying Open No. Hei 2-149823 discloses a technique in which fiber plates are used in place of the glass substrates for making the optical system compact. However, the fiber plate is so expensive that the overall image display device may be very costly.
To solve the above shortcomings, the Japanese Patent Lying Open No. Hei 2-55386 has disclosed a technique of providing means for forming an erected image with the same magnification, for example, a rod lens array between the CRT and the reflective type liquid crystal light valve. With this forming means, the image of the CRT is formed on the photoconductive layer. The thickness of the glass substrate does not bring about a parallax and the optical system is made compact.
This technique, however, does not disclose a concrete acceptance angle of a rod lens array, a size of the image display means, a pitch of display pixels, or degree of parallel light. Further, at the filing time (August, 1988) of the patent, no liquid crystal panel which has a higher resolution and is smaller than the CRT had been developed. Hence, the technique provides no concept of using a liquid crystal panel in place of the CRT.
The U.S. Pat. No. 5,083,854 discloses a technique of locating a rod lens array between the liquid crystal panel and the reflective type liquid crystal light valve, magnifying a pixel opening of the liquid crystal display panel, and forming the magnified pixel portion onto the photoconductive layer. Moreover, the Japanese Lying Open No. Hei 2-149823 discloses a technique of locating the fiber plate on the glass substrate located on the writing light source side of the reflective type liquid crystal light valve.
However, the prior art disclosed in the Japanese Patent Lying Open No. Hei 2-55386 uses the CRT. The reduced CRT offers a lower resolution because of the smaller diameter of an electron beam forming the image and the bleeding of a fluorescent material. Hence, the critical size of the CRT suitable to the HDTB is 5 inches. It means that the reduction of the optical system is limited. Further, since the CRT is effected by the geomagnetism, the image may be distorted or a conversion shift may take place in the three plate type projection using the reflective type liquid crystal light valve for each of the RGB colors.
The prior art disclosed in the U.S. Pat. No. 5,083,854 is required to correspond the rod lens array to the liquid crystal display panel in one-to-one manner on the principle of the operation. Both of the pitches have to coincide with each other. In recent days, however, the liquid crystal panel is developed to have a pixel pitch of 100 .mu.m or less. The high-definition liquid crystal panel corresponding to the HDTV is recently developed to have a pitch of 30 .mu.m or less (SID '93 digest pp. 888 to 886), while the now commercially available rod lens array is manufactured to have a pitch of 1 mm or less (Selfoc Lens produced by Japan Sheet Glass Company, Limited, for example). It is quite difficult to technically make both of them coincide with each other. If this difficult is overcome, one rod lens serves to prevent double image Formation of pixels adjacent to each other and thereby restrict the degree of parallel light. Hence, the writing light is made quite dark. If the pixel is shifted out of the rod lens, a moire pattern may take place. Hence, both of them are required to be accurately positioned. The positioning is quite troublesome.
The technique of locating the fiber plate on the glass substrate on the writing light source of the reflective type liquid crystal light valve, as disclosed in the Japanese Patent Lying Open No. Hei 2-149828, enhances the cost of the image display device because the fiber plate itself is quite costly.